With the improvement of the operation speed of a personal computer, spread of the network infrastructure, an increase in data storage capacity and a reduction in prices of data storage in recent years, there has been increasing occasions of obtaining and viewing the simpler electronic information of documents and images and the like which used to be provided in a form of a printed copy in the conventional manner.
A conventional liquid crystal display, a CRT and, in recent years, a light emitting type display such as an organic EL display have been used as a means for viewing electronic information. However, when the electric information is text information, the viewing device must be gazed at for a comparatively long period of time, and this action is not very human friendly.
Generally, the known disadvantages of the light-emitting type display include visual fatigue caused by flicker, poor portability, restricted posture for viewing so as to put the line of sight on a still image, and an increase in power consumption when viewed for a long period of time.
One of the known devices to measure these disadvantages is a (memory-type) reflective display that uses the external light consuming no power to maintain images. However, this kind of device does not provide satisfactory performances for the following reasons.
The method of using the polarizing plate such as a reflective liquid crystal has a low reflectivity of about 40 percent, and therefore has a problem when displaying white color. Many of the manufacturing methods of the components are not very easy or simple. Further, since the polymer dispersed liquid crystal display requires a high voltage, and the contrast of the obtained image is not sufficiently high because it uses the difference in the refractive indexes of organic materials. The polymer network liquid crystal display requires a high driving voltage and a complicated TFT circuit for a better memory characteristic. The display element using an electrophoresis method requires a high voltage of 10 volts or more. There is a concern about the durability resulting from aggregation of electrophoretic particles.
The display methods known to overcome the drawbacks of the aforementioned types include the electrochromic display element (hereinafter abbreviated as “EC type”) and electrodeposition type using solution and deposition of metals or metallic salts (hereinafter abbreviated as “ED type”).
The EC type is capable of full-color display by a low voltage of 3 volts or less, and is characterized by simple cell configuration and excellent white color quality. Similarly, the ED type is capable of driving by a low voltage of 3 volts or less, and is characterized by simple cell configuration, excellent black-and-white contrast and black color quality. Various methods of those types have been proposed (see Patent Literatures 1 through 5 for example).
As described above, the EC type and ED type are capable of being driven by a low voltage of 3 volts or less, and is characterized by simple cell configuration, excellent display quality and paper-like white and crisp black quality.
Patent Literature 1: International Publication No. WO2004/068231
Patent Literature 2: International Publication No. WO2004/067673
Patent Literature 3: U.S. Pat. No. 4,240,716
Patent Literature 4: Japanese Registration Patent No. 3428603
Patent Literature 5: Japanese Unexamined Patent Application Publication No. 2003-241227